Elemental sulfur and micronutrient elements have known utility as soil amendments and plant nutrients. However, contemporary application techniques result in the unavoidable micronutrient loss due to elution by irrigation or rain water. This loss occurs when the micronutrient metals are added as or converted to soluble forms which can be dissolved and drawn into the soil along with water movement and thus beyond the levels with which they are available as plant nutrients. This migration, particularly of boron and the heavy metals such as zinc, copper, molybdenum and manganese, has recently been associated with the pollution of ground water and streams. Furthermore, if present in excess, these materials accumulate at toxic levels in treated plants.
Obviously, however, these metals must be applied in soluble forms or as compounds that can be converted to soluble forms to assure their availability to the crop. Thus it is necessary to strike a balance between the mobility and immobility of these materials in order to, on the one hand, afford their availability to the crop at the desired rate while avoiding toxic accumulations in the plants, ground water pollution or, simply, excessive micronutrient loss to the ground water.
The problem is further complicated by the influence of soil composition per se on compound mobility. For instance, many of these compounds and particularly the more abundant less expensive compounds such as the oxides, hydroxides, sulfides and carbonates are only slightly soluble at best under conditions of application. Their solubility is even further reduced in basic, calcareous soils thus requiring high application rates to assure availability.
This problem might be overcome to some extent by changing the gross soil chemistry, i.e., by adding sufficient acid to increase soil pH and improve micronutrient mobility. It might also be possible to combine the micronutrient compounds with a composition such as elemental sulfur or ammonium nitrate which might reduce soil pH locally, i.e., in the immediate vicinity of the sulfur or ammonium nitrate particle thereby increasing micronutrient mobility by either simply lowering soil pH or possibly even converting the compound to a more soluble form, e.g., the sulfate. While this procedure would have the advantage of increasing metal availability, it suffers from the disadvantage that the micronutrient compound, once mobilized, could be readily swept out of the immediate vicinity of the plants by irrigation or rain water. This condition exists even if the compound is applied on the surface of the soil amendment (sulfur, ammonium nitrate, etc.) since those surfaces would be periodically washed by water applied to the soil.
It is therefore one object of this invention to provide an improved combination soil amendment and slow release micronutrient source. It is another object to provide a particulate, highly porous combination of elemental sulfur and plant micronutrients. Another object is the provision of a porous, particulate sulfur-micronutrient combination which acts as a slow release micronutrient source and is particularly adapted to improving the availability of micronutrients in calcareous soils while reducing micronutrient loss. It is another object to provide a composition and method for making micronutrients available to plant crops at a controlled rate while minimizing micronutrient loss in the ground water or the accumulation of excessive micronutrient levels in the plant crop.
These objectives are accomplished by combining the micronutrient with highly porous sulfur particles having substantial internal surface area so that a substantial amount, and preferably a principal amount, of the micronutrient is located either within the sulfur matrix or on the interior surface area of the sulfur particle. These combinations expose a higher surface area, for a given weight of sulfur and micronutrient, on which elemental sulfur and the micronutrient are intimately associated and are exposed to oxidizing action. I have found that the mobile forms of micronutrient created or maintained in this environment immediately adjacent the interior surface area are not immediately swept away by elution due to irrigation or rain water penetration. On the contrary, the nutrients are retained within the interior of the sulfur particle.
The benefit of increased surface area might be obtained within finely divided sulfurs of smaller particle size. Unquestionably, similar surface areas could be obtained. However, aside from the problems of handling fine powders, the increased surface area would be present at the exterior of the sulfur particles. Thus any mobile micronutrient compound could be readily eluted by ground water. The mobile micronutrients would be readily swept past the effective zone of plant nutrition and lost or, if made available to the plants upon the addition of water by irrigation or rain, would result in a slugging effect, i.e., the induction of high concentrations of micronutrients into the crops on a periodic basis depending on the irrigation and/or rain cycles.
Therefore, in accordance with one embodiment of this invention, improved slow release soil amendments and micronutrient sources are produced by impregnating porous sulfur particles having an internal pore volume of at least about 0.04 cc per gram and an internal surface area of at least about 20 square centimeters per gram, with an aqueous solution of a water soluble micronutrient compound. The solution is preferably concentrated so that the impregnation step will require as little water as possible. Concentrated solutions allow more efficient solution use. Desired micronutrient levels can be reached by applying only the amount of solution required to either fill the pores of the sulfur particles or introduce the required amount of micronutrient, whichever is less.
In a preferred embodiment, the micronutrient metal or compound is incorporated into the sulfur particles by combining molten sulfur with the metal or a water soluble compound and thereafter converting the combination to the porous particulate form. A preferred procedure for this conversion is described in my U.S. Pat. No. 3,769,378 and, in essence, involves mixing high velocity streams of water and molten sulfur in a turbulent zone created by the intersection of the two streams.